Surgical sutures made from polypropylene monofilaments have been successfully used by the medical profession for more than ten years. The advantages exhibited by polypropylene sutures include the following:
(a) They pass easily through tissue;
(b) They resist breakdown and do not promote infection;
(c) They have minimal reaction with tissue;
(d) They have high tensile strength; and
(e) They maintain their in vivo tensile strength over extended periods.
The preferred polypropylene suture used in the medical profession today is described by Listner in U.S. Pat. No. 3,630,205.
As good as the current polypropylene sutures are, there is room for improvement. In particular, it would be desirable to increase the compliance, limpness, or felxibility of polypropylene sutures, especially with the larger suture sizes, in order to make them easier to handle and to improve their knot security. The problem is that previous efforts to accomplish this have occasioned a concomitant undesirable decrease in strength properties.
One approach to the provision of sutures having the desirable properties of polypropylene sutures, with the added feature of being more compliant, has been to employ random ethylene-propylene copolymers containing a small amount of polymerized ethylene, as is disclosed by Menezes et al. in U.S. patent application Ser. No. 432,487, filed Oct. 4, 1982, entitled "Ethylene-Propylene Copolymer Sutures", and assigned to the same assignee as this application, and abandoned Nov. 28, 1984.
The extrusion of polymers into monofilaments is well established technology, as is illustrated by the Listner patent cited above. The typical procedure is to extrude the monofilament and pass it to a first station such as a godet. The monofilament is quenched, either by air or by a liquid quench bath, between the extruder and the first station. The monofilament may be "drawn down" slightly between the extruder and the first station, but the draw down ratio or "jet stretch" (i.e., ratio of speed of take up at the first station to extrusion speed) will rarely exceed about 4X. After the first station, the monofilament is oriented by drawing. In order to achieve uniform orientation, the degree of draw achieved between the first station and the next station should be the "natural draw ratio". The natural draw ratio is defined as the ratio of take-up speed to let-off speed at which an undrawn filament will spontaneously draw via a "neck" formation when subjected to an axial extension above the filament's yield point. The natural draw ratio is a moderately narrow range of draw ratios that is dependent, in part, upon factors such as nature of polymer, molecular weight, drawing conditions, especially temperature, and the like. If the draw ratio used is less than the natural draw ratio, undrawn areas of monofilament will be present, and if the draw used in a single drawing step is above the natural draw ratio, microscopic inhomogeneities such as voids and fibrils will be formed.
This invention is directed to the provision of improved polypropylene surgical monofilaments that are significantly more compliant than prior polypropylene surgical monofilaments (as is evidenced by significantly lower Young's Modulus values), but which retain, at least to a large degree, the excellent properties of prior polypropylene surgical monofilaments, and to a method for producing the improved sutures. The process for producing the surgical monofilaments of this invention involves additional orientation in one or more added drawing stages beyond that achieved by the initial draw that is carried out at the natural draw ratio, followed by a heat relaxation step with annealing.
This invention is based upon the discovery that an increase in the draw ratio during orientation increases the tensile strength more than the modulus of the filament, and that an increase in the shrinkage allowed during annealing decreases the modulus more than the tensile strength. Thus, an increase in both the draw ratio and the allowed shrinkage during annealing results in a filament of lower modulus at a given level of tensile strength.